This application proposes to use hybridomas to produce monoclonal antibodies against antigens on human leukemia cells and/or their non-malignant lymphoid counterparts. Monoclonal antibodies recognizing antigens unique to or associated with human T lymphocytes will be used to a) evaluate malignant lymphocytes and determine whether biologically distinct groups of patients can be defined, b) study normal T lymphocyte differentiation by evaluating normal lymphocytes and their precursors in bone marrow, "regenerating" bone marrow, thymus, and peripheral blood for the expression of such antigens, and c) study the functional characteristics of normal peripheral blood lymphocytes expressing such antigens. Monoclonal antibodies will also be produced against the acute lymphoblastic leukemia antigen (ALLA). This reagent will be used to study the expression of ALLA on leukemic cells and to determine whether this antigen is present on early lymphoid precursors present in "regenerating" bone marrow. Finally, long term bone marrow cultures ("Dexter" cultures) will be used to study the relationship between ALLA-plus cells and pre-B lymphocytes in "regenerating" bone marrow.